Mastodon can trace the origins of their fourth album to the pavement in front of the Mandalay Bay casino in Las Vegas. That's where the Atlanta metal band's guitarist-vocalist, Brent Hinds, was attacked and cracked his head open during a drunken altercation following the MTV VMAs in September 2007. Hinds suffered brain hemorrhaging, and then, as he recovered, he was plagued by vertigo.

"I was dizzy for eight months," he says, hanging out at a studio in the band's hometown, along with guitarist Bill Kelliher, bassist-vocalist Troy Sanders and drummer Brann Dailor. "During those months, I just sat with my acoustic and a little marijuana, and wrote all the music. I wanted a more melodic, easy-listening situation, and that definitely came from having head trauma."

Mastodon's last set, 2006's Blood Mountain, was their major-label debut, and it brought them up from the metal underground to a wider audience, earning them a Grammy nomination and MTV spins for "Colony of Birchmen." The new album continues the trend. "We wanted to make something a little broader," says Dailor. "A classic-rock-sounding record."

Those impulses led Mastodon to producer Brendan O'Brien (Bruce Springsteen, Pearl Jam). They were initially introduced to O'Brien backstage at a Springsteen show by E Street Band drummer Max Weinberg, whose son is a Mastodon fan. "I asked Bruce, 'How was Brendan to work with?' " says Dailor. "He said, 'He's the only person I've ever trusted with my music.' I was like, 'Well, you're the Boss.'"

O'Brien brought a relaxed vibe to the studio and helped flesh out the album's classic-rock heart. "Quintessence" adds spacey washes of synthesizers and vocal harmonies to the brawny riffs and piercing guitar lines. The thrashing "Divinations" comes on like Master of Puppets-era Metallica, only with soaring vocals and a bright chorus, and "Oblivion" inserts a bluesy guitar solo into a sinister, multitempo, prog-metal epic.

The finished album will contain only seven songs, and, like previous Mastodon outings, will be a concept record. After past efforts focused on fire, water and earth, the new disc will explore air, ghosts and the ethereal world. The group has talked about eventually combining the releases into a box set called The Elements.

"Musically and lyrically, this is the record we'd be writing if we weren't going to exist tomorrow," says Sanders. "We don't feel like we have to do a certain type of record to please a certain group. We're free to change."